Manipulation of solid, semi-solid or liquid materials

ABSTRACT

By applying two or more electrical fields (DC, AC, pulsed) of different characteristics to an electrode array on e.g. the wall of a treatment cell, particles suspended in liquid in the cell may be manipulated as desired on a microscopic scale, in particular by exploiting the dielectrophoretic properties of the particles. The particles may be solid, semi-solid or liquid, and may be of simple materials or may be biological particles such as whole cells or fragments thereof.

This is a continuation of application Ser. No. 7/952,456, filed Jul. 29,1992 , now abandoned, which is a continuation of PCT/GB91/00122, Jan.29, 1991.

BACKGROUND OF THE INVENTION

This invention relates to the manipulation of solid, semi-solid orliquid materials in liquid media.

A wide variety of commercial processes involve the use of liquid mediahaving solid, semi-solid or liquid particles suspended in them. Theparticles may vary very widely from inert inorganic materials through toreactive materials, and organic or biological structures such as cellsor parts of cells.

It has been known for some time that particles of these various typesmay be caused to move within a liquid medium by the use of a non-uniformelectric field, and the basic phenomenon of dielectrophoresis has beenextensively discussed, for example see "Dielectrophoresis", CambridgeUniversity Press, 1978 by H. A. Pohl and Chapter 6 of "Dielectric andElectronic Properties of Biological Materials", John Wiley & Sons 1979by Ronald Pethig.

Recently the application of dielectrophoresis has been suggested in thearea of materials classification: the construction of a so-called"optical dielectrophoresis spectrometer" is described in Burt, Al-Ameen& Pethig, "An optical dielectrophoresis spectrometer for low-frequencymeasurements on colloidal suspensions", Journal of Physics, Section E,Scientific Instrumentation, Volume 22 (1989) pages 952 to 957.

That paper, and the related paper "Applications of a New opticalTechnique for Measuring the Dielectrophoretic Behaviour ofMicroorganisms", Price, Burt and Pethig Biochimica et Biophysica Acta964 (1988) pages 221 to 230, disclose the use of interdigitatedelectrodes deposited on a dielectric substrate to cause movement ofsuspended particulates by the dielectrophoretic effect.

SUMMARY OF THE INVENTION

Most previous work has been directed to the characterization ofmaterials by taking appropriate measurements of their electricfield-induced properties. Another major application is the use ofpositive dielectrophoretic forces to align biological cells betweenelectrodes prior to their electrofusion, as described by W. M. Arnoldand U. Zimmermann ("Electric Field Induced Fusion and Rotation ofCells", Biological Membranes 5, 389-454, 1984). Also, a method andapparatus for dielectrophoretic manipulation of chemical species hasbeen described by J. S. Batchelder (U.S. Pat. 4,390,403, Jun. 28, 1983).This method employs DC non-uniform electrical fields to manipulate oneor more chemicals within a multi-electrode chamber so as to promotechemical reactions between the chemical species. The applied voltage maybe periodically reversed in sign to decrease ionic shielding effects(see column 3, line 62 to column 4, line 3). The manipulation of thechemicals is controlled by positive dielectrophoretic forces resultingfrom differences in the dielectric constants of the chemical species.

In previous works of S. Masuda, M. Washizu and I. Kawabata "Movement ofBlood Cells in Liquid by Nonuniform Travelling Field", IEEE Transactionson Industry Applications, Volume 24 (1988) pages 217 to 222, blood cellswere caused to move under the influence of a non-uniform travellingelectric field. This field was generated by applying two,fixed-frequency, multiphased, voltage signals, related by having thesame frequency and amplitude, to a series of parallel electrodes.Likewise, the rotating electric field described by W. M. Arnold and U.Zimmermann and employed to cause rotation of a single cell, is generatedusing either a single, phase-split, voltage signal or synchronized,identical, voltage pulses. The present invention utilizes two or moreelectric fields that are generated using electrically independentvoltages that do not share the same frequency of oscillation.

We have now found that if two or more non-uniform electric fields ofdiffering frequencies are imposed, simultaneously or sequentially, on asuspension of particles of one or more than one type in a liquid, usingappropriate electronic control, various reactions may be stimulated tooccur in the particles in the liquid.

Thus, in accordance with a first broad aspect of the invention, a methodof promoting reactions between particles suspended in a liquid isprovided, the particles being of uniform type. The method, comprisesapplying two or more independent non-uniform electrical fields ofdifferent frequencies to the liquid from an electrode array in such afashion as to provoke or promote the desired reaction.

The term reaction as used herein is to be interpreted broadly ascovering various chemical, biochemical and physical interactions, andlarge scale manipulations such as separation followed by recombination,optionally with a treatment being selectively applied to one componentof a multicomponent system while so separated from the othercomponent(s).

Whereas previous work (e.g. Batchelder) has employed differences in thedielectric constant of the manipulated chemical species to control thedesired reactions, we have found that, in addition to varying thedielectric constant, varying the electrical conductivity of either orboth the suspended particles and the suspending medium provides afurther degree of control. In this connection (and throughout thisspecification) the term dielectric constant is used to refer to the realpart of the complex permittivity.

The particles which may be used may be of animate or inanimate materialand they may be colloidal or of some other nature.

With appropriate choice of the dielectric constant and electricalconductivity of either or both the suspended particle and suspendingmedium, both positive and negative dielectrophoretic forces may beemployed as the manipulating agent. Preferably, in carrying out themethod of the invention, at least one of the electrical fields is chosento effect a negative dielectrophoretic force on some only of theparticles in the liquid. By appropriate choice of electrode geometry, itis possible to achieve at appropriate regions within the electrodegeometry, regions where particular species of particle segregate orparticular particle agglomerations occur. In order to enable adequatequantities of suspension to be treated, the electrodes may take the formof a repeating pattern array, or. e.g. two electrodes may be comblikeand inter-engaged with each extending part of each lying between twoneighboring such parts of the other.

The present invention also provides apparatus for carrying out themethod of the invention including a treatment cell including anelectrode array, means for feeding a suspension of particles in a liquidto the treatment cell, and means for removing liquid from the cell.First means connected to electrodes in the cell are adapted to generatea first non-uniform electrical field within the cell, and second meansconnected to electrodes in the cell and adapted to generate a secondnon-uniform electrical field within the cell having a frequency whichdiffers from that of the non-uniform first electrical field.

Preferably, the electrode array is mounted on en external wall of thetreatment cell. Such an arrangement preferably includes, as part of theliquid removing means, perforations in the external wall of the cellbearing the electrodes, the perforations being so located that, when theelectrodes are appropriately electrically activated relative to theparticles in the liquid medium in the cell and to the liquid mediumitself, the liquid and particles drawn off through the perforations willdiffer from the general bulk characteristics of the suspension ofparticles in the liquid in the cell.

In the description of specific illustrated apparatus which follows, forsimplicity of expression, reference is made to applying signals toelectrodes. It should be understood that in order to create theappropriate electric fields, the signal is, in practice, applied acrossa pair of electrodes, one of which may be a reference electrode or anextensive surface area `ground plate` or the like.

Depending upon the nature of the particles in the suspension, the natureof the suspension itself, and the nature of the applied electricalfields to the electrode array, the particles may be caused to aggregatetowards regions of an electrode or a number of electrodes oralternatively detach themselves from such electrode(s) and/or aggregatetowards regions away from the electrodes. These phenomena may beexploited in a wide variety of applications to manipulate the particlesin suspension, e.g. to provide the directed assembly of structures fromsuch suspended particles, or to provide separation from a mixture ofsuspended particles of different types, to provide characterization ofparticular particle types or to promote a reaction between particles oftwo or more different types.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings will serve to illustrate the invention in moredetail by way of example. In the drawings:

FIGS. 1A(1)-1A(3) and 1B(1)-1B(3) and 1B(1)(a) show top, side and endviews of two different examples of electrode geometries which may beused in the method of the present invention,

FIGS. 2 and 3 show diagrammatically the movement of a particle using theelectrode array of FIG. FIGS. 1A(1)-1A(3) ,

FIG. 4 shows diagrammatically a more complex situation,

FIGS. 5A, 5B and 5C show the electrode geometry of FIG. 1B(1)-1B(3) andexamples of particle aggregation at regions on the electrodes andregions away from the electrodes,

FIG. 6 shows diagrammatically two different particle types collected atthe same time using both positive and negative dielectrophoretic forces,

FIG. 7A shows the random distribution of two particle types in theregions near two independent pairs of electrodes,

FIG. 7B shows the resulting distribution of two particle types after theindependent electrode pairs have been energized by twocharacteristically different voltages, and

FIG. 8 shows a block diagram of apparatus which can be used to carry outthe method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1A(1)-1A(3) show in plan, side and endviews, respectively, one wall of a treatment cell which can be used tomanipulate a suspension consisting of one or more particle types. Theapparatus comprises an array of electrodes 1 fabricated on a suitablesubstrate 2 that forms the wall or surface of the treatment cell. Eachelectrode can be individually and independently energized by any form ofelectrical signal via electrical connectors 3. The electrodes 1 may bein direct electrical contact with the particle suspending liquid orseparated from it by an appropriate material. For the purpose ofillustration the electrodes 1 in FIG. 1A(1)-1A(3) are shown having arectangular geometry, but other geometries may be used depending on theparticle characteristics and the desired effect to be achieved.

FIG. 2 shows the array of electrodes 5-8 and a test particle 4 suspendedin a liquid adjacent to the array of electrodes 5-8. By applying anappropriate electrical signal to electrode 5, the test particle 4 can bedielectrophoretically attracted towards electrode 5. This effect isenhanced by applying, at the same time, another electrical signal toelectrodes 6 and 7 such that test particle 4 is dielectrophoreticallyrepelled from electrodes 6 and 7. The electrical signals are applied toelectrodes 5, 6 and 7 until test particle 4 becomes immobilized atelectrode 5 or reaches a desired locality in the region of electrode 5.The particle can then be further moved by applying, for example,electrical signals which repel the particle from the region of electrode5 and attract it towards electrode 8.

FIG. 3 shows particle 4 after it has been dielectrophoreticallymanipulated to the region of electrode 8 and also shows another particle9 located at an electrode 10. Particle 9 may or may not have the samedielectric and conductive properties as particle 4. Particles 4 and 9may be positioned alongside each other by dielectrophoretically movingeither or both of them. Bringing particles 4 and 9 (and any others inlike fashion) into association may be effected for the purpose ofconstructing larger building blocks or for inducing a specific chemical,biological or electrochemical reaction between them. This exampledescribes the bringing together of particles but, in the more generalcase, the apparatus may be employed to manipulate particles into anydesired positions relative to each other.

FIG. 4 shows a collection of particles composed of particle types 4 and9, which in this case possess differing bulk and/or surface electricalproperties. By applying an electrical signal to electrodes 5 and 8 thatdiffers from the signal applied to electrodes 6 and 7, and by anappropriate choice of signal characteristics (i.e. waveform, magnitudeand frequency) as well as the suspending medium characteristics (e.g.pH, dielectric constant, conductivity and specific density) particletypes 4 and 9 may be physically separated from each other with, forexample, particle type 4 collecting near electrodes 5 and 8 and particletype 9 collecting near electrodes 6 and 7. The electrical signalsapplied to electrode pairs 5 and 8 and 6 and 7, or any other electrodecombination, may be applied continuously or intermittently, and at thesame or differing times, in order to achieve the desired separation. Theparticle types may then be removed separately from the treatment cell bydrawing off the particle suspending fluid through perforations locatednear the electrodes, having first released the desired particle typefrom the electrodes either by removing the electrical signal used tocollect them or, if strong electrode adhesion occurs, releasing them byapplying electrical signals of appropriate characteristics to theelectrodes. The extent of particle collection at the electrodes can becontinuously assessed using an optical monitoring technique as describedby Burt, Al-Ameen and Pethig in the Journal of Physics, Section E,Scientific Instrumentation, Volume 22 (1989) pages 952 to 957, and thesubsequent release of particles can likewise be monitored by an opticalprobe at the electrodes and also downstream of the perforations.

FIG. 1B(1)-1B(3) shows, in plan, side and end views, respectively,another electrode geometry which can be used to manipulate a suspensionconsisting of one or more particle types. For the purpose ofillustration, the electrodes 11, 12 in FIG. 1B(1)-1B(3) are shown havinga castellated, interdigitated, rectangular, geometry, but othergeometries may be used depending on the particle characteristics and thedesired effect to be achieved.

FIG. 5A shows diagrammatically a section of an array of interdigitated,castellated, electrodes 11 and 12 after the application of a voltagebetween electrodes 11 and 12. Two different particle types 13 and 14have been aggregated as long chains at the outer tips of the individualelectrode castellations, as a result of both particle types experiencinga positive dielectrophoretic force.

FIG. 5B shows diagrammatically the same type of electrode configurationas in FIG. 5A, where two different particle types 13 and 14 have beenaggregated into regions of the upper electrode surfaces, away from theelectrode sides, as a result of experiencing a negativedielectrophoretic force.

FIG. 5C shows diagrammatically the same type of electrode configurationas in FIGS. 5A and 5B, where two different particle types 13 and 14 havebeen directed into forming triangular-shaped aggregations in the regionsbetween the electrode castellations away from the electrode sides, as aresult of experiencing a negative dielectrophoretic force.

The following Examples will serve to illustrate the invention.

EXAMPLE 1

A cell was taken with an array of interdigitated, castellated,electrodes substantially as shown in FIGS. 1B(1)-1B(3). Eachcastellation was 20 microns wide, 40 microns deep, about 0.1 micronhigh, spaced at 80 micron centers, the interdigitated electrode rowsbeing 80 microns apart. The entire array had sixty electrodes in eachrow, and was around 5mm long. The array was located on one wall of acell having 7.5 cubic mm of internal volume.

A suspension in a medium of 280 mM mannitol in deionized watercontaining as suspended particles Micrococcus lysodeikticus (ellipsoidsaround 2 microns long and 0.5 microns across) was added in equal amountto a suspension of latex particles (1.27 micron diameter) in deionizedwater. The concentration of suspended particles was such that theoptical absorbance at a wavelength of 635 nm, and 1 cm path length, was,in each case, 1.61 (cf deionized water). The conductivity of theMicrococcus suspension was 11.4 micro-Siemens per cm, whilst that of thelatex particle suspension was 2.1 micro-Siemens per cm. When evenlydistributed, the amount of interaction between the latex particles andthe Micrococcus was very small.

On application of a voltage of 4 V p/p sinewave at a frequency of 100kHz, the latex and Micrococcus, particles aggregated as long chains atthe outer tips of the individual electrode castellations in a similarmanner to that shown in FIG. 3 of the 1988 Biochimica et Biophysica Actapaper of Price, Burt and Pethig, and as also schematically shown in FIG.5A. The term "positive dielectrophoresis" as used herein is to beinterpreted broadly as this form of particle aggregation in which theparticles move towards the areas of higher field strength. On removal ofthe applied voltage, the latex and Micrococcus particles separated fromeach other and became dispersed in the suspending medium. This processof bringing the latex and Micrococcus particles into intimate contactwith each other, and then letting them separate, can be repeated manytimes.

On application of a voltage of 4 V p/p sinewave at a frequency in therange between 100 Hz and 1 kHz, the latex and Micrococcus particlesaggregated at regions of the upper electrode surfaces, away from theelectrode sides, as illustrated in FIG. 5B. This form of particleaggregation, where the particles are directed away from high electricfield regions at electrode edges, is not the normal form of positivedielectrophoresis, and herein is to be interpreted broadly as negativedielectrophoresis.

EXAMPLE 2

A cell was taken of the same form as that in Example 1, but eachelectrode castellation was 80 microns wide, 80 microns deep, about 0.1micron high, spaced at 160 micron centers and with the interdigitatedelectrode rows being spaced at 160 microns apart. The entire electrodearray had sixty electrodes in each row, and was around 1.0 cm long. Thearray was located on one wall having an a cell of internal of 30 cubicmm.

A suspension in a medium of 280 mM mannitol in deionized water wasprepared containing as suspended particles equal numbers of live Brewersyeast cells and dead (autoclaved) Brewers yeast cells to an opticalabsorbance of about 0.8 at a wavelength of 635 nm for 1 cm path length.On application of a voltage of 20 V p/p sinewave at a frequency range of100 Hz to 20 MHz, both the live and dead yeast cells experienced apositive dielectrophoretic force and collected at the outer tips of theelectrode castellations.

On increasing the electrical conductivity of the mannitol suspendingmedium, by the addition of potassium chloride, to a conductivity of 150micro-Siemens per cm, then, depending on the frequency of the voltageapplied to the electrodes, the two cell types could each be made toexperience either a negative or positive dielectrophoretic force. Forexample, when a voltage of 20 V p/p sinewave at a frequency of 10 kHzwas applied to the electrodes, the dead yeast cells were observed tocollect at the outer tips of the electrode castellations (as shown inFIG. 5A) as a result of experiencing a positive dielectrophoretic force,whilst the live yeast cells were directed by a negativedielectrophoretic force into a triangular-shaped aggregation in theregions between the electrode castellations away from the electrodesides, as shown in FIG. 5C. The overall collection of the live and deadyeast cells is similar to that shown in FIG. 6, where the live yeastcells are labelled as particle type 15 and the dead yeast cells arelabelled as particle type 16. On the other hand, if a voltage of 20 Vp/p sinewave at a frequency of 10 MHz was applied to the electrodes,then the live yeast cells experienced a positive dielectrophoretic forceand collected in the form of FIG. 5A, whilst the dead cells experienceda negative dielectrophoretic force and aggregated in the triangular formas shown in FIG. 5B.

EXAMPLE 3

A cell was taken with an array of interdigitated, castellated,electrodes of the same geometry and dimensions as that used in Example 1above. A suspension in a medium of deionized water was prepared,containing as suspended particles two types of latex particles ofdiameter 1.27 microns. The first type of latex particle was coated withthe antibody raised in rabbit against horseradish peroxidase, whilst thesecond type of latex particle was coated with horseradishperoxidase-labelled antibody raised in swine against the horseradishperoxidase antibody raised in rabbit. The final suspension of electricalconductivity 4.1 micro-Siemens per cm was formed by mixing together inequal volumes a suspension of latex particle of the first type, ofoptical density 0.8 at a wavelength of 635 nm at a 1 cm path length,with a suspension of latex particle of the second type of opticaldensity 0.54 at a wavelength of 635 nm at a 1 cm path length.

On application of a voltage of 4 V p/p sinewave at a frequency of 1 kHzto the electrodes, both latex particle types experienced a negativedielectrophoretic force and rapidly aggregated at regions of the upperelectrode surfaces, away from the electrode sides, as shown in FIG. 5B.This aggregation brought both types of latex particle into close contactwith each other, and greatly accelerated the rate of interaction betweenthe two antibody types coated on the latex particles. On removing theapplied voltage to the electrodes, a significant number of the latexparticles were observed to be bound together as a result of thedielectrophoretically-induced interaction of the latex particles.

EXAMPLE 4

A cell was taken with an electrode geometry similar to that shown inFIGS. 7A and 7B, where the electrode pair 17 can be energized by anapplied voltage separately and independently from electrode pair 18. Theseparation between the two electrodes forming electrode pair 17 was 104microns, and likewise for the two electrodes forming electrode pair 18.Each electrode element had a width of 32 microns, and a height of 0.1micron, and the two electrode pairs were spaced 130 microns apart fromeach other.

A suspension in a medium of 280 mM mannitol in deionized water wasprepared, which contained, as suspended particles, equal numbers of liveBrewers yeast cells 19 and dead (autoclaved) Brewers yeast cells 20 toan optical absorbance of about 0.8 at a wavelength of 635 nm and a 1 cmoptical path length. To this suspension medium was added potassiumchloride of sufficient concentration to increase the electricalconductivity of the medium to 150 micro-Siemens per cm. A voltage of 20V p/p sinewave at a frequency of 10 kHz was applied to electrode pair 17of FIGS. 7A and 7B, and at the same time a voltage of 20 V p/p sinewaveat a frequency of 10 MHz was applied to electrode pair 18. The liveBrewers yeast cells 19 were observed to be attracted by a positivedielectrophoretic force towards, and to collect at, the electrode pair18 energized by the voltage oscillated, and to be repelled by a negativedielectrophoretic force from the regions around the electrode pair 17energized by the voltage oscillated at 10 kHz. The dead Brewers yeastcells 20, on the other hand, were repelled by a negativedielectrophoretic force from the regions around the electrode pair 18energized by the voltage oscillated at 10 mHz, but were attracted by apositive dielectrophoretic force towards, and collected at, theelectrode pair 17 energized by the voltage oscillated at 10 kHz. Thedistribution of live 19 and dead Brewers yeast cells 20 before and afterapplication of the 10 kHz and 10 MHz oscillating voltages is shown inFIGS. 7A and 7B, respectively. As can be seen, a spatial separation ofthe live Brewers yeast cells 19 from the dead Brewers yeast cells 20 isaccomplished by this procedure.

EXAMPLE 5

A cell was taken with an array of interdigitated, castellated, electrodepair of the same geometry and dimensions as that used in Example 1above. Two types of glass beads of nominal diameter 1.0 micron wereused. The first type of glass bead was shaken in 5% aminopropyltriethoxysilane in dry acetone for 3 hours, then washed and dried at 70degrees Centigrade. After drying, the beads were shaken in a 5% solutionof nitrophenyl ester of d-biotin in chloroform. This procedure resultedin the first type of glass bead being coated with a film of d-biotin.The second type of glass bead was shaken in a solution containing 1 mgper ml of avidin in phosphate buffered saline at pH 7.7 for 20 minutes.The treated beads were then washed three times in phosphate bufferedsaline solution. This procedure resulted in the second type of glassbead being coated with a film of avidin. The first and second type 1 and2 were then separately suspended in a solution of potassium chloride ofelectrical conductivity 3.5 micro-Siemens per cm.

The final suspension was formed by mixing together in equal volumes asuspension of the first type of glass bead, of optical density 0.8 at awavelength of 635 nm at 1 cm optical path length, with a suspension ofthe second type glass bead of optical density 0.8 at a wavelength of 635nm at a 1 cm optical path length.

On application of a voltage of 6 V p/p sinewave at a frequency of 800 Hzto the electrodes, both glass particle types experienced a negativedielectrophoretic force and aggregated at regions of the upper electrodesurfaces, away from the electrode sides, similar to that shown in FIG.5B. On removal of the applied voltage, a significant number of the glassbeads were observed to be firmly bound together as a result ofavidin-biotin complexes being formed between the surfaces of both typesof glass beads.

What is claimed is:
 1. A method of promoting a desired reaction betweenparticles suspended in a liquid, said method comprising stepsof:applying, at a first frequency, a first non-uniform electrical fieldto a region of said liquid from an electrode array; and applying, at asecond frequency, a second non-uniform electrical field to said regionof said liquid from said electrode array substantially simultaneouslywith said application of said first non-uniform electrical field, saidsecond non-uniform field being independent of said first non-uniformelectrical field; said first non-uniform electrical field and saidsecond non-uniform electrical field promoting said desired reactionusing dielectrophoretic forces on said particles; and at least one ofsaid first frequency and said second frequency being chosen to effect anegative dielectrophoretic force on only some of said particlessuspended in said liquid.
 2. A method according to claim 1, furthercomprising a step of:modifying at least one of an electricalconductivity and a dielectric constant of at least one of said liquidand said particles to further promote said desired reaction.
 3. A methodof promoting said desired reaction between said particles suspended insaid liquid according to claim 1, wherein:said electrode array includesparallel electrode pair.
 4. A method of promoting said desired reactionbetween said particles suspended in said liquid according to claim 1,said method comprising further steps of:determining said first frequencyto be a first value which causes a negative dielectrophoretic force onsaid only some particles suspended in said liquid; and determining saidsecond frequency to be a second value which causes a positivedielectrophoretic force on others of said particles other than said onlysome particles suspended in said liquid.
 5. A method of promoting saiddesired reaction between said particles suspended in said liquidaccording to claim 1, said method comprising further stepsof:determining said first frequency to be a first value which causes anegative dielectrophoretic force on said only some particles suspendedin said liquid; and determining said second frequency to be a secondvalue which causes a negative dielectrophoretic force on others of saidparticles other than said only some particles suspended in said liquid.6. A method of promoting said desired reaction between said particlessuspended in said liquid according to claim 1, said method comprisingfurther steps of:determining said first frequency to be a first valuewhich causes a negative dielectrophoretic force on said only someparticles suspended in said liquid; and determining said secondfrequency to be a second value which causes a further negativedielectrophoretic force on said only some particles suspended in saidliquid.
 7. A method of promoting said desired reaction between saidparticles suspended in said liquid according to claim 1, wherein:saidelectrode array is interdigitated and creates field traps wherein atleast some of said particles suspended in said liquid agglomerate usingnegative dielectrophoretic forces.
 8. A method of promoting said desiredreaction between said particles suspended in said liquid according toclaim 7, wherein:said interdigitated electrode array is castellated. 9.A method of promoting said desired reaction between said particlessuspended in said liquid according to claim 7, wherein:saidinterdigitated electrode array includes a repeating pattern array.
 10. Amethod of promoting said desired reaction between said particlessuspended in said liquid according to claim 7, wherein:saidinterdigitated electrode array forms an inter-engaged comb-like patternwith an extending portion of a first electrode lying between twooppositely extending and neighboring portions of a second electrode. 11.A method of promoting said desired reaction between said particlessuspended in said liquid according to claim 7, wherein:said electrodearray is mounted on an external wall of a treatment cell, said treatmentcell including perforations in said external wall to allow removal ofparticles effected by said desired reaction through said perforations.12. A method of promoting a desired reaction between particles suspendedin a liquid, said method comprising steps of:applying, at a firstfrequency, a first non-uniform electrical field to a region of saidliquid from an electrode array; and superimposing, at a secondfrequency, a second non-uniform electrical field to said region of saidliquid from said electrode array substantially simultaneously with saidapplication of said first non-uniform electrical field, said secondfrequency being different from said first frequency, said secondnon-uniform field being independent of said first non-uniform electricalfield; wherein said first non-uniform electrical field and said secondnon-uniform electrical field promote said desired reaction usingdielectrophoretic forces on said particles.
 13. A method of promoting adesired reaction between particles suspended in said liquid, said methodcomprising steps of:introducing a volume of said liquid, including saidsuspended particles, into a treatment cell provided with electrodes;applying a first electrical signal to said electrodes to generate aredistribution of said particles under an effect of a firstdielectrophoretic force; applying, simultaneously with said firstelectrical signal, a second electrical signal to said electrodes, afrequency of said second electrical signal being different from afrequency of said first electrical signal, wherein said frequency ofsaid second electrical signal is chosen taking into account a dielectricconstant and electrical conductivity of said particles and said liquidto produce a second dielectrophoretic force acting on said particles,said second dielectrophoretic force being different from said firstdielectrophoretic force.
 14. A method of promoting a desired reactionbetween particles suspended in said liquid according to claim 13,wherein one of said first electrical signal and said second electricalsignal applies a negative dielectrophoretic force on only some of saidparticles suspended in said liquid.
 15. A method of promoting a desiredreaction between particles suspended in said liquid according to claim13, said method further comprising:modifying one of said electricalconductivity and said dielectric constant of one of said liquid and saidparticles.
 16. A method of promoting a desired reaction betweenparticles suspended in said liquid according to claim 13, said methodfurther comprising:determining said frequency of said first electricalsignal to be a first value which causes a negative dielectrophoreticforce on only some of said particles suspended in said liquid; anddetermining said frequency of said second electrical signal to be asecond value which causes a positive dielectrophoretic force on othersof said particles other than said only some particles suspended in saidliquid.
 17. A method of promoting a desired reaction between particlessuspended in said liquid according to claim 13, said method furthercomprising:determining said frequency of said first electrical signal tobe a first value which causes a negative dielectrophoretic force on onlysome of said particles suspended in said liquid; and determining saidfrequency of said second electrical signal to be a second value whichcauses a negative dielectrophoretic force on others of said particlesother than said only some particles suspended in said liquid.
 18. Amethod of promoting a desired reaction between particles suspended insaid liquid according to claim 13, said method furthercomprising:determining said frequency of said first electrical signal tobe a first value which causes a negative dielectrophoretic force on onlysome of said particles suspended in said liquid; and determining saidfrequency of said second electrical signal to be a second value whichcauses a further negative dielectrophoretic force on said only someparticles suspended in said liquid.
 19. A method of promoting a desiredreaction between particles suspended in a liquid, said particlesincluding first particles and second particles, said method comprisingsteps of:introducing a volume of said liquid, including said suspendedparticles, into a treatment cell provided with electrodes; applying, ata first frequency, a first electrical signal to said electrodes togenerate a redistribution of positions of said particles under an effectof a first dielectrophoretic force; applying, at a second frequencydifferent from said first frequency, while said particles occupy saidredistribution of positions, a second electrical signal to saidelectrodes, said second frequency being chosen taking into account adielectric constant and electrical conductivity of said particles andsaid liquid to produce a second dielectrophoretic force acting on saidparticles, one of said first and second electrical signals acting toapply a negative dielectrophoretic force on only said first particles,and the other of said first and second electrical signals acting toapply only one of a negative dielectrophoretic force and a positivedielectrophoretic force on both said first particles and said secondparticles.
 20. A method of promoting a desired reaction betweenparticles suspended in said liquid according to claim 19, said methodfurther comprising:modifying one of said electrical conductivity andsaid dielectric constant of one of said liquid and said particles.
 21. Amethod of promoting a desired reaction between particles suspended in aliquid, said particles including first particles and second particles,said method comprising steps of:introducing a volume of said liquid,including said suspended particles, into a treatment cell provided withelectrodes; applying, at a first frequency, a first electrical signal tosaid electrodes to generate a redistribution of positions of saidparticles under an effect of a first dielectrophoretic force acting onsaid first particles and said second particles, said firstdielectrophoretic force being one of a positive dielectrophoretic forceand a negative dielectrophoretic force; applying, at a second frequencydifferent from said first frequency, while said particles occupy saidredistribution of positions, a second electrical signal to saidelectrodes, said second frequency being chosen taking into account adielectric constant and electrical conductivity of said particles andsaid liquid to produce a second dielectrophoretic force acting on saidparticles, said second dielectrophoretic force being a positivedielectrophoretic force acting on one of said first particles and saidsecond particles, and said second dielectrophoretic force being anegative dielectrophoretic force acting on the other of said firstparticles and said second particles.
 22. A method of promoting a desiredreaction between particles suspended in said liquid according to claim21, said method further comprising:modifying one of said electricalconductivity and said dielectric constant of one of said liquid and saidparticles.